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1
Chapter 30 Detergents
30.1 Introducing detergents
30.2 Structure of detergents
30.3 Properties of detergents
30.4 Making detergents
30.5 Washing powder
30.6 Comparing soaps and soapless detergents
30.7 Problems associated with use of detergents
CONTENTS OF CHAPTER 30
2
30.1 INTRODUCING DETERGENTS
We use detergents every day, in one form or another. Like
plastics and alkanols, most detergents are made from petroleum
products.
30.1 INTRODUCING DETERGENTS
3toothpaste
shampoo
soap body lotion face cleaning lotion
washing powder dish-washing liquid car wash
Figure 30.1 Detergents for different cleaning jobs.
30.1 INTRODUCING DETERGENTS
4
WHAT IS A DETERGENT?
TYPES OF DETERGENTS
There are two types of detergents:
Soapless detergents (or synthetic detergents)
Soapy detergents (or soaps)
A DETERGENT is a substance which helps water to clean things
better.
30.1 INTRODUCING DETERGENTS
5
Figure 30.2
Soapless detergents include washing
powders, washing-up liquids, shampoos
and hair conditioners. They are called
‘soapless’ because they contain no soap.
30.1 INTRODUCING DETERGENTS
Figure 30.3
Soapy detergents include bath soaps,
laundry soaps and liquid soaps.
6
30.2 STRUCTURE OF DETERGENTS
30.2 STRUCTURE OF DETERGENTS
GENERAL STRUCTURE OF DETERGENT PARTICLES
Detergents are usually sodium (or potassium) salts of long-chain
organic acids.
The detergent anion consists of two parts:
(1) An ionic group (the ‘head’)
(2) A hydrocarbon chain (the ‘tail’)
7
Figure 30.4 General structure of a detergent anion.
Detergent anions therefore attract to both water and oil. This
dual nature explains two important properties of detergents — the
wetting property and the emulsifying property.
30.2 STRUCTURE OF DETERGENTS
8
Structure of soapless detergent particles
Two common soapless detergents are:
Sodium alkylbenzene sulphonate
30.2 STRUCTURE OF DETERGENTS
9
Sodium alkyl sulphate
30.2 STRUCTURE OF DETERGENTS
10
Structure of soap particles
Soaps are sodium (or potassium) salts of long-chain alkanoic
acids. The ionic head of soaps is always a carboxylate group
(–COO). A common soap is sodium stearate:
30.2 STRUCTURE OF DETERGENTS
11
A30.1
30.2 STRUCTURE OF DETERGENTS
12
30.3 PROPERTIES OF DETERGENTS
30.3 PROPERTIES OF DETERGENTS
AS A WETTING AGENT
Water has a high surface tension.
Figure 30.6
A pond skater. It can
‘walk’ on water, which
has a high surface
tension.
13
A detergent reduces the surface tension of water. As a result
of this, water spreads over the surface and wets it more easily.
A detergent thus acts as a wetting agent.
30.3 PROPERTIES OF DETERGENTS
14
tap water
detergent solution
Figure 30.7
A detergent increases the wetting power of water. Tap water does not wet this piece
of cloth easily, but a detergent solution does.
30.3 PROPERTIES OF DETERGENTS
15
AS AN EMULSIFYING AGENT
Oil and water do not mix. An oil-water emulsion is unstable. On
standing, the tiny oil droplets rapidly join together and grow larger
to form a separate oily layer again.
30.3 PROPERTIES OF DETERGENTS
16
water
oil
Figure 30.8
Oil and water do not mix.
30.3 PROPERTIES OF DETERGENTS
17
Figure 30.9
Shaking a mixture of water and oil and allowing it to stand.
30.3 PROPERTIES OF DETERGENTS
18
30.3 PROPERTIES OF DETERGENTS
An oil/water emulsion is unstable.
19
Figure 30.10
Shaking a mixture of water and oil (with a little detergent added) and allowing it to stand.
30.3 PROPERTIES OF DETERGENTS
20
An oil-water emulsion is stabilized by a detergent.
30.3 PROPERTIES OF DETERGENTS
Figure 30.11
How detergent anions arrange themselves in an oil-water mixture:
(b) after shaking (c) negatively charged oil
droplets repel each
other.
(a) before the mixture is
shaken
21
30.3 PROPERTIES OF DETERGENTS
An oil/water emulsion is stabilized by a detergent.
22
30.3 PROPERTIES OF DETERGENTS
To test properties of a detergent.
23
CLEANSING ACTION OF DETERGENTS
foam
oil/water emulsion stabilized by detergent
Figure 30.12
A detergent solution stabilizes an
oil/water emulsion.
30.3 PROPERTIES OF DETERGENTS
24
30.3 PROPERTIES OF DETERGENTS
25
DETERGENTS are cleansing agents. They are surfactants
(surface active agents). They work by reducing the surface
tension of water, enabling it to wet things more effectively, and by
emulsifying grease.
In general, ionic groups joined to hydrocarbon chains having
12 to 20 carbon atoms have good detergent properties.
30.3 PROPERTIES OF DETERGENTS
26
A30.2
(a) C, F and G.
Detergents are sodium or potassium salts of long-chain
organic acids (usually with number of carbon atoms between
12 and 20).
A is an ester. B is an alkanoic acid. D has too few carbon
atoms, while E has too many, H is a magnesium salt of
alkanoic acid, so they do not possess good detergent
properties.
(b) C and F. (G is a soapless detergent as its anionic group is
— OSO3, not — COO).
30.3 PROPERTIES OF DETERGENTS
27
30.4 MAKING DETERGENTS
30.4 MAKING DETERGENTS
MAKING SOAPLESS DETERGENTS
Soapless detergents are manufactured from hydrocarbons
obtained from petroleum.
28
MAKING SOAPS
Soaps are made from animal fats (e.g. beef and mutton fat) or
vegetable oils (e.g. palm oil and coconut oil).
Figure 30.14
The palm oil from
these palm trees can
be used to make soap.
30.4 MAKING DETERGENTS
29
Fats and oils are naturally occurring triesters. In general, the
formula of fats and oils can be represented as:
30.4 MAKING DETERGENTS
30
Figure 30.15
(a) General formula of
fats and oils.
(b) Model of a fat/oil
molecule.
30.4 MAKING DETERGENTS
31
There are two basic processes in making soaps:
Saponification
Salting-out of soap
Saponification
Fat is hydrolysed (broken down by water) in alkaline solution to
give a soap. The process is called saponification.
fat / oil + sodium hydroxide glycerol + soap
30.4 MAKING DETERGENTS
32
A30.3
(a) No. Paraffin oil is a mixture of hydrocarbons, not esters.
(b) No. Only alkalis can saponify fats and oils.
Salting-out of soap
After saponification is complete, much of the soap dissolves in the
solution. To get the soap out , add a concentrated sodium chloride
solution. This process is called salting-out of soap:
conc. NaCl(aq)
Soap(aq) soap(s)
30.4 MAKING DETERGENTS
33
30.4 MAKING DETERGENTS
To prepare a soap.
34
A30.4
(1) Wear safety spectacles.
(2) Cover the beaker with a watch glass (to prevent solution from
spurting out of the beaker).
(3) Heat the beaker gently.
30.4 MAKING DETERGENTS
35
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
30.6 COMPARING SOAPS AND SOAPLESS
DETERGENTS
COMPARING CLEANING ABILITIES IN
SOFT WATER AND HARD WATER
Water may be soft or hard. Soft water contains no or only very
small concentrations of dissolved calcium and/or magnesium ions.
Hard water contains appreciable concentrations of calcium and/or
magnesium ions.
36
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
To compare action of soap and soapless detergent in soft water
and hard water.
37
Experiment results show that soaps form a lather easily in soft
water; they hardly form any lather in hard water. Instead, they give
a sticky insoluble substance called scum. On the other hand,
soapless detergents form a lather easily in both soft water and
hard water.
A30.5
(a) (i) Yes (ii) No.
(b) (i) Yes (ii) Yes.
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
38
ADVANTAGES AND LIMITATIONS OF SOAPS
Advantages
Soap has several advantages as a detergent:
(1) It cleans very well in soft water.
(2) It is non-toxic to water life.
(3) It is biodegradable (i.e. can be broken down by bacteria).
Therefore it will not cause foaming in sewage works and
rivers.
(4) It is only mildly alkaline (with pH between 8 and 9). Thus it
seldom causes skin allergy.
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
39
Limitations
Soap has the following limitations:
(1) Soap is only slightly soluble in water. Thus it is seldom used
in washing machines.
(2) It does not work well in hard water. Taking sodium stearate
as an example of soap,
Ca2+(aq) + 2CH3(CH2)16COO–(aq) (CH3(CH2)16COO)2Ca(s)
Mg2+(aq) + 2CH3(CH2)16COO–(aq) (CH3(CH2)16COO)2Mg(s)
stearate ion (scum)
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
40
(a) (b)
Figure 30.19
(a) Soap in soft water — lather is
formed.
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
(b) Soap in hard water — no lather is
formed. Note the scum formed.
41
(3) Soap cannot be used in strongly acidic solutions. Taking
sodium stearate as example,
CH3(CH2)16COO–(aq) + H+(aq) CH3(CH2)16COOH(s)
stearate ion (from acid) stearic acid
(with no detergent properties)
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
42
Figure 30.20
Soap in strongly acidic solution — insoluble alkanoic acid is precipitated out.
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
43
A30.6
(a) Hard water. Sea water contains magnesium salts in addition
to sodium chloride.
(b) No. Sea water is hard water. The calcium and/or magnesium
ions present would react with soaps to form scum.
SOLVING THE SOAP PROBLEM
(1) Remove the hardness and acidity of water. An effective water
softener is washing soda, sodium carbonate-10-water
(Na2CO3 •10H2O).
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
44
Ca2+(aq) + CO32–(aq) CaCO3(s)
Mg2+(aq) + CO32–(aq) MgCO3(s)
Phosphates are also used as softeners.
(2) Use soapless detergents instead of soaps.
ADVANTAGES OF SOAPLESS DETERGENTS
(1) Soapless detergents do not have the limitations of soaps.
They do not form scum with hard water.
(2) Soapless detergents are made from petroleum products, not
from fats and oils.
(3) Soapless detergents can be ‘tailor-made’ to suit a particular
cleaning problem.
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
45
Figure 30.22
Specially designed soapless detergents each suited for a particular purpose.
30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS
46
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
30.7 PROBLEMS ASSOCIATED WITH USE
OF DETERGENTS
PROBLEMS ASSOCIATED WITH
NON-BIODEGRADABLE DETERGENTS
Detergents used in the early 1950s were non-biodegradable. This
is because the early detergent particles contained branched
hydrocarbon chains.
47
Figure 30.23 The structure of an early soapless detergent.
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
48
Figure 30.24
This river had a persistent thick foam caused by non-biodegradable detergents.
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
49
Soapless detergents nowadays are biodegradable. They
contain straight (unbranched) hydrocarbon chains.
Figure 30.25
The structure of a biodegradable soapless detergent.
(Note the straight
hydrocarbon chain.)
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
50
A30.7
Hydrocarbon chains in soaps are straight (i.e. unbranched).
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
51
PROBLEMS ASSOCIATED WITH BIODEGRADABLE
DETERGENTS
Even biodegradable detergents affect rivers and lakes. Firstly,
because they are biodegradable, bacteria would use up dissolved
oxygen in water.
Secondly, commercial detergents usually contain phosphate
additives. Phosphates are plant nutrients, causing rapid growth of
algae in rivers and seas. This is believed to be one of the causes
for ‘red tides’ in Hong Kong waters.
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
52
Figure 30.26
Detergents may not be harmful to the environment, but the additives may.
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
53
Figure 30.27 Rapid growth of algae due to rich phosphates in water.
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
54
Figure 30.29
Fish killed by red tides.
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
55
EEFECT OF DETERGENTS ON SKIN
Most detergent solutions have pH values between 5 and 9.
Detergents with pH values outside this range may cause skin
allergy.
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
56
Figure 30.31 Detergents with too high or too low a pH are harmful to the skin.
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
57
A30.8
30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS
58
SUMMARY
1. A detergent is a substance which helps water to clean things
better.
A detergent has cleaning properties because it can act as a
wetting agent and an emulsifying agent.
2. There are two types of detergents:
Soapless detergents — made from petroleum
products
Soaps — made from animal fats or plant oils
3. Detergents are usually sodium (or potassium) salts of long-
chain organic acids.SUMMARY
59
SUMMARY
4. (a) General structure of a detergent anion:
(b) General structure of a soapless detergent anion:
60
SUMMARY
(c) General structure of a soap anion:
5. Soaps can be made by reacting animal fats or plant oils with
sodium hydroxide. Two processes are involved:
Saponification
fat / oil + sodium hydroxide glycerol + soap
Salting-out of soap
61
SUMMARY
6. Soft water contains no or only very low concentrations of
dissolved calcium and/or magnesium ions.
Hard water contains appreciable concentrations of calcium
and/or magnesium ions.
7. Soaps work well in soft water but not in hard water. Soapless
detergents work well in both soft water and hard water.
8. Problems associated with the use of detergents:
Detergents may cause skin allergy.
Detergents go down the drain into the sewage
system and eventually to rivers or seas. Bacteria in
water use up oxygen during the decomposition of these
detergents. This would kill water life.
62
SUMMARY
Many detergents contain phosphate additives. The
phosphates are nutrients for algae. This may lead to ‘red
tides’ and death of water life.